1. Field of the Invention
Embodiments of the present invention generally relate to the fabrication of integrated circuits and more particularly to the thermal processing of a substrate using laser anneal.
2. Description of the Related Art
Thermal processing is required in the fabrication of integrated circuits formed in silicon wafers and other substrates, such as glass panels for displays. Required temperatures may range from relatively low temperatures, e.g., less than 250 degrees Celsius, to temperatures of 1000 degrees Celsius to 1400 degrees Celsius, and may be used for a variety of processes such as dopant implant annealing, crystallization, oxidation, nitridation, silicidation, and chemical vapor deposition as well as others.
For the very shallow circuit features required for advanced integrated circuits, it is desirable to minimize total thermal budget of a process while still achieving the required thermal processing. The thermal budget is considered the total time at which it is necessary for a substrate to remain at high temperatures to achieve the desired processing. In many applications, this time may be very short. In addition, it is often advantageous to only heat a very thin region at the surface of a layer.
Laser thermal processing (LTP) is one method of thermal processing that has a reduced thermal impact on the bulk of the substrate while facilitating the melting and recrystallization of a thin layer at the surface of the substrate. LTP utilizes short pulses of laser radiation to thermally anneal and activate the dopants in semiconductors as part of the process of forming a semiconductor device, such as a metal oxide semiconductor (MOS) device. Dopant activation via LTP is achieved by melting a thin layer of semiconductor material to diffuse the dopants within the molten region. During cooling, the molten material re-crystallizes, fixing the dopants into the lattice sites where they remain electrically active. To ensure that only a thin layer of the substrate surface is melted, the duration of the laser pulse is very short, e.g., on the order of 5 to 100 ns. The quantity of energy that must be delivered to produce the desired annealing result is on the order of about 0.1 J/cm2 and greater.
Because of the relatively large quantity of energy required and the very short time interval in which it must be applied to the substrate, pulse lasers are the typical delivery mechanism utilized for the melting and recrystallization of the surface of a substrate. Other methods, such as lamps, cannot provide such a high energy input in such a short time interval.
In thermal processing, it is important to uniformly heat the structure being processed. One drawback to the use of pulse lasers for annealing substrates is the non-uniform illumination of a target area on the substrate due to coherence effects, such as interference fringes and laser speckle. In addition to having the ability to deliver a high energy pulse for a short time, lasers also produce light waves that often have high temporal and spatial coherence-though the degree of coherence depends strongly on the exact properties of the laser. Spatial coherence of laser beams may manifest itself as speckle patterns and diffraction fringes on the target area, which are caused by constructive and destructive interference of the largely coherent light waves. Hence, one point in a target area may be illuminated with essentially no light due to destructive interference while another point a few micrometers away may have overly bright illumination due to constructive interference, resulting in non-uniform illumination, and therefore non-uniform thermal processing, of the target area.
It is possible to spatially modulate the speckle and interference patterns of a coherent light beam over time so that, when averaged over a suitable time interval, each point on the target area of the light beam will experience uniform energy input. For example, a rotating light diffuser may be placed between the coherent light source and the target area. As the diffuser moves relative to the light source, e.g., via rotation or translation, the speckle and interference patterns present on the target area will also move and, when averaged over a time interval of appropriate duration, result in uniform illumination of the entire target area.
Because the duration of an LTP laser is so short, methods of laser modulation known in the art are unable to produce any significant effect on a coherent light source on the nanosecond timescale. To wit, mechanical motion of a lens and/or diffuser over such a brief time is problematic since spinning a diffuser or moving a mirror fast enough to provide a benefit is mechanically impracticable. For example, a galvo mirror system may operate on the kHz timescale, whereas to produce one period of modulation over 5 ns, a method operating at 200 MHz is required. Other known methods, such as acousto-optic methods, are also too slow, since they operate on the 10's of kHz timescale and do not approach the MHz regime. Acousto-optic deflection of a coherent light source relies on the high-speed variation of the index of refraction of a light-transmitting material when bulk acoustic waves are passed therethrough.
Therefore, there is a need for a method and an apparatus that can reduce coherence effects present during LTP.